Voltage regulators keep the output voltage regulated even if a difference between the input voltage and the output voltage is very low (e.g., 100 mV). If the input voltage is sufficiently high, then the output voltage is at a nominal level and the voltage regulator is operating in a closed loop. However, if the input voltage drops, then the voltage regulator starts to operate in an open loop, which is also referred to as the dropout mode.
Current consumption of a voltage regulator can be significant when operating in the dropout mode. An example voltage regulator 10 is illustrated in FIG. 1 and includes an input terminal 12 to receive an input voltage VIN, an output terminal 14 to supply an output voltage VOUT, and a power transistor 20 having a first conduction terminal 22 coupled to the input terminal 12, a second conduction terminal 24 coupled to the output terminal 14, and a control terminal 26.
A differential amplifier 30 has a first input 32 to receive a voltage reference VREF, and a second input 34 to receive a feedback signal VFB corresponding to the output voltage VOUT. An output 36 of the differential amplifier 30 provides a drive signal VDIFF based on a difference between the voltage reference VREF and the feedback signal VFB.
A driver 50 includes an impedance device 52 coupled to the control terminal 26 of the power transistor 20, and a driver transistor 54. The driver transistor 54 has a first conduction terminal 55 coupled to the control terminal 26 of the power transistor 20, and a control terminal 57 receiving the drive signal VDIFF from the differential amplifier 30 so as to vary a bias current IBIAS to the control terminal 26 of the power transistor 20.
Since the output 58 of the driver 50 is coupled to the power transistor 20, a voltage formed across the impedance device 52 represents VGS of the power transistor. As the load current ILOAD of the voltage regulator 10 changes, VGS of the power transistor 20 also changes. The relation between the load current ILOAD and VGS is given by a transfer function of the power transistor 20. The transfer function is valid when the power transistor 20 is operating in the saturation region. This corresponds to the voltage regulator 10 operating in the closed loop. Since the impedance device 52 is operating between the control terminal 26 and the first conduction terminal 22 of the power transistor 20, the bias current IBIAS of the driver 50 depends on the load current ILOAD.
If the difference VDROP between the input voltage VIN and the output voltage VOUT is sufficiently high, the power transistor 20 stays in the saturation region and VGS of the power transistor is relatively low (e.g., below 1 V). This results in a low bias current IBIAS within the driver 50. If the voltage difference VDROP becomes too low so that the voltage regulator 10 is not able to maintain operating in the closed loop, then the power transistor 20 passes to a linear region. This corresponds to the voltage regulator 10 operating in the dropout mode.
In the dropout mode, the dependence between the load current ILOAD and VGS of the power transistor 20 is no longer given by the transfer function of the power transistor, and VGS can reach a very high level. In fact, the driver 50 can pull down the control terminal 26 of the power transistor 20 to near ground GND, and VGS of the power transistor 20 can approach the input voltage VIN. Since the driver 50 operates over VGS of the power transistor 20, the bias current IBIAS can reach a very high level. In the case of VIN=5 V and a resistive load of the driver transistor 54, the bias current IBIAS can be 5 times higher then the bias at the maximum load current ILOAD. This is valid even if the load current ILOAD is 0 when current consumption of the voltage regulator 10 should be minimal.
As an example, if the voltage level of a battery used to power an electronic device starts to discharge, then the voltage regulator 10 within the electronic device passes from operating in the closed loop to operating in the dropout mode. Operating in the dropout mode results in a significant change in the operating point of the voltage regulator 10, especially in the VGS of the power transistor 20, which can increase up to the input voltage VIN.
For the above illustrated voltage regulator 10, the bias current IBIAS in the driver 50 of the power transistor 20 depends on the VGS of the power transistor 20. If the VGS increases in the dropout mode, then the bias current IBIAS increases as well. For a battery powered electronic device, this means that when the battery becomes discharged and the voltage regulator 10 passes to the dropout mode, even more current starts to sink. This is an undesired behavior and can compromise the electronic device operating time or can even threaten battery safety. Consequently, there is a need for controlling current consumption of the voltage regulator 10 when operating in the dropout mode.